This invention deals generally with wastewater treatment and more specifically with a wastewater purification system for small flow applications.
As environmental standards and regulations become more stringent, it is less likely that any wastewater treatment system dependent on only a septic tank, other than those serving only a single residence, can meet the required standards. In fact, even a septic tank for a single residence may not meet the regulations if the soil conditions or background nitrate-nitrogen levels are not suitable for the most common wastewater xe2x80x9ctreatmentxe2x80x9d system, a drain field which simply spreads the wastewater over a large area below ground surface.
The processes generally used to treat the wastewater from septic tanks and other small flow systems requiring treatment beyond reduction in organic strength are generally referred to as xe2x80x9ctertiary xe2x80x9d treatment systems, and their goal is to reduce the biological nutrient content in the wastewater. One of the most significant biological nutrients is nitrogen, and nitrogen is a particular problem in agricultural areas, which are also the most likely regions to lack the population density to warrant the construction of full capability municipal sewage disposal facilities.
However, without proper treatment, nitrogen contamination of groundwater that supplies wells may result in nitrate levels exceeding the drinking water standard for nitrogen, which is 10 milligrams per liter. Since the contaminated groundwater also ultimately flows into groundwater fed streams, the elevated nitrogen levels can also result in steam eutrophication and a general degradation in water quality.
The removal of nitrogen from wastewater is actually a two step process. The first step, called nitrification, requires conversion of the nitrogen in ammonia to nitrogen in nitrates. This is accomplished by the use of aerobic bacteria; nitrosomonas and nitrobacter, that are already present in domestic wastewater. These bacteria grow and multiply in the presence of dissolved oxygen when the oxygen has a concentration of greater than 1 milligram per liter, and the bacteria use the nutrients present in the wastewater as their food source. The natural action of the bacteria converts ammonia nitrogen into nitrate nitrogen.
The second stage of the nitrogen removal process involves the conversion of the nitrate nitrogen into gaseous nitrogen and is called denitrification. This is accomplished by other bacteria, many varieties of which also naturally exist in the wastewater. These bacteria perform the conversion of nitrates to gaseous nitrogen, but only under the condition that the dissolved oxygen in the wastewater is below 1 milligram per liter. When this condition is met, the bacteria generate gaseous nitrogen from the nitrates, and the nitrogen is released harmlessly into the atmosphere. The reduction of dissolved oxygen in the wastewater is a result of the bacteria respiration, so essentially the denitrification process can be automatically triggered by the previous process of nitrification.
The most common systems presently used to treat wastewater are modified suspended growth processes such as the activated sludge process, which can operate conventionally or with a sequential batch reactor. For the nitrification step, these systems add air into the wastewater, either before or after the solids are settled out, to dissolve air into the liquid. This adds oxygen to the wastewater and simultaneously reduces the organic strength of the waste. The aeration is then stopped, and the natural respiration action of the bacteria present in the wastewater causes the dissolved oxygen level in the wastewater to drop. When the dissolved oxygen level falls below 1 milligram per liter, the denitrification step begins.
The success of this technique in suspended growth reactors requires very close control of the dissolved oxygen levels during each phase of the treatment. In larger facilities where trained full-time operators and automated process sensors, particularly oxygen sensors, are used to control the operating parameters, and the waste flow and strength are relatively stable, the process works reasonably well. However, small community, commercial, and on site systems attempting to achieve removal of nitrogen face a more difficult task. Wastewater composition and quantity are highly variable, and full-time certified operators familiar with the process and the sophisticated process control equipment are rarely employed.
For installations dealing with relatively small quantities of waste, it would be very beneficial to have a simple wastewater treatment system which operates without highly trained personnel and without sophisticated and expensive equipment.
The present invention is an apparatus which uses and encourages the natural bacteriological processes so that the entire process of removal of nitrogen from wastewater needs only minimal control.
For purposes of the description of the invention, and because it is the most common situation in which the invention is used, it is assumed that the primary treatment of the waste is by a septic tank. The septic tank, or other equivalent primary treatment, functions to remove the solid waste, typically by settling the solid waste to the bottom of a tank. Such a primary treatment device should, of course, provide reliable and efficient removal of solid waste, but a second apparatus for further solid removal is also desirable.
Typically, this secondary treatment is a prefilter device, many of which are already conventionally used for wastewater treatment. The function of such a prefilter device is to further reduce the solids in the wastewater to prevent clogging of the subsequent treatment apparatus and bypass device. This device may be a flow diversion valve or simply a smaller diameter pipe than the primary pipe which delivers septic tank effluent to the recirculation tank.
After the solid material is satisfactorily removed, the remaining wastewater, the effluent, is put through a recirculating packed media filter which includes a recirculating tank. The recirculating packed media filter has a fine gravel media or coarse sand which supports a bacterial xe2x80x9cfixed filmxe2x80x9d. The fixed film consists of a variety of bacteria which are already present in the wastewater being treated and form a layer or film of bacteria cells which is attached to the sand or gravel in the recirculating packed media filter.
The wastewater is treated as it flows over the fixed film, and the bacteria extract nutrients necessary for their respiration and growth from the wastewater, thus purifying the wastewater stream. In order to maintain aerobic conditions in the recirculating packed media filter the wastewater is fed into the recirculating packed media filter by pulse dosing, intermittently feeding, the wastewater from a recirculating tank back into the recirculating packed media filter. Between the dosings of wastewater, the wastewater drains from the recirculating packed media filter permitting air to enter into the spaces within the media and provide oxygen to maintain aerobic conditions.
The recirculating packed media filter is followed by a subsurface flow constructed wetland into which the initially treated wastewater flows for denitrification. The subsurface flow constructed wetland provides the anoxic environment, with less than 1 milligram per liter of oxygen, necessary to convert the nitrate based nitrogen in the effluent flowing from the recirculating packed media filter into nitrogen gas.
To aid in the denitrification process some wastewater entering the nitrogen removal system of the invention is diverted around the recirculating packed media filter and flows directly into the subsurface flow constructed wetland. This bypass flow is in the range of 5 to 25% of the entire wastewater flow, and the ratio is dependent upon the input wastewater strength and the limit for nitrogen concentration in the system""s output. The bypass flow into the subsurface flow constructed wetland serves the dual purposes of providing a carbon source for the denitrification process and accelerating the transition from aerobic conditions, with oxygen levels above 1 milligram per liter, to the anoxic conditions required for the denitrification.
The invention thereby accomplishes its purpose of nitrogen removal with only two major components, a recirculating packed media filter and a subsurface flow constructed wetland. This simple and relatively passive apparatus requires significantly less operator attention, and yields energy savings of at least 50% compared to conventional suspended growth systems.